Surface mounting involves a planar mounting of components to a circuit board having a previously-applied conductive pattern. The components may be individual surface-mounted components, flip chips, tape automated bonding formats, and so forth. The conductive pattern for most components is obtained by a printing process that involves depositing a layer of attachment media, such as solder paste, onto a stencil or screen. Solder is forced through openings in the stencil or screen onto pads located on the circuit board below using a conventional squeegee blade. Alternatively, pressurized squeegee heads having two squeegee blades can be used to apply the solder with the use of internal pressure, by either rolling it onto the stencil or forcing it through the stencil openings. This is in contrast to the conventional squeegee blade in which solder paste is applied as it rolls in front of a single blade. With either type of system, these processes produce a full or complete printing of the entire circuit board in one step, after which the components are placed on the circuit board in the usual manner.
Full printing, however, provides limited flexibility in terms of being able to apply components at different times, as the surface over which the squeegee blade or head travels needs to be substantially planar. Failure of a die after it and all of the other components have been added to the circuit board can be costly. As a result, many manufacturers use various package processes, such as a controlled collapse chip connection (C4) process, to produce a package containing the die. In this way burn-in and testing of the die can take place prior to mounting it on the board. Such packages are costly to produce, however, and failure of the die after it is part of the package is also expensive. Such cost concerns have caused many manufacturers to limit their designs so as to produce smaller packages, eliminate large or heavy parts, avoid double-sided circuit boards, and so forth.
An alternative “two-step” printing process is sometimes used when different pitches of print are desirable. The first step involves a first printing or fluxing for the die. A second or “partial” printing is then performed using a step-down stencil of varying thickness to block the pre-printed (or fluxed) die locations. Following the second printing, components (including surface mount components and the die) are placed on the circuit board in the usual manner. Typically, the entire process is performed in a clean room, although a clean room is only necessary for placing the die on the circuit board. However, the same problem of costly die failure exists.
Another means by which a “partial” print is obtained involves dispensing solder paste through the needle of a syringe. This method is very slow, however, because the paste is dispensed onto only one pad or “land” at a time. Clogging of needles is also a common problem with this method. Special dispense heads can be used to reduce the cycle time, although this increases the costs of the process significantly. Furthermore, due to the high cost of these processes, the complexity of the circuit board or module design is limited.
For the reasons stated above, there is a need in the art for a more convenient and economic method of manufacturing circuit boards using partial printing.